Last Tuesday, Google announced it was expanding its fledgling Ad Sitelinks beta to all advertisers whose ads meet a quality threshold. Ad Sitelinks, like its organic Sitelinks counterpart, enables advertisers to designate links to pages within their site to appear below their listing. In the paid version of the program, an advertiser can select up to 10 links, in order of preference, for Google to consider. Depending on the performance of the individual links and their relation to the searcher’s query, Google will show as many as four at a time (or as few as none), in the order it deems most effective.

Also noteworthy is that Ad Sitelinks are configured at the campaign level. While this integrates happily with AdWords conversion tracking, it may prove a bane to advertisers using non-Google keyword-level tracking.

For a Sitelinks-enabled ad to be eligible for display, it must appear as the top paid listing and already have a substantially higher click-through rate than competing ads. In other words, expect ads for brand terms to be granted extra elbow room, but not ads for other keywords.

Advertisers who rely on brand keywords to generate a large number of conversions should embrace Ad Sitelinks as a way to differentiate their ads from their competitors’, as well as from affiliates, resellers, and research sites–the fortunate triumvirate Google allows to use competitor trademarks in its ad copy. In addition, Sitelinks offer a way to loosen the 95-character constriction of an AdWords ad, and to test multiple promotions and offers simultaneously.

But more skeptical advertisers will wonder whether Sitelinks are even necessary at all. For example, Yahoo’s policy of allowing only the holder of a trademark to bid on its brand terms–through bemoaned (in some instances, legitimately) by resellers–removes the haze created by Google’s more nuanced trademark rules. Yahoo’s reasoning is unabashedly simple: an ad for a brand’s official site is the most relevant listing for a search performed for that brand. Why can’t Google make the same assumption?

A Sitelinks product manager tells me Google saw a 20%–30% lift in click-through rate on Sitelinks-enabled ads during beta testing. Oddly, though, Google has observed few of these additional clicks occurring on the Sitelinks themselves, but instead on the ad headline. (Sitelinks clicks are reported together with clicks on the ad headline via AdWords, although I’m told this may change in the product’s future incarnations.) If more searchers are clicking on the headline of Sitelinks-enabled ads, and not on the Sitelinks themselves, what, exactly, are they responding to?

The hope is that these extra clicks are coming at the expense of competing ads, but it’s likely that many are simply shifting from an advertiser’s similar-looking organic listings. And as the number of advertisers using Ad Sitelinks grows–and thus the number of ads that appear not to be ads–so, too, will the number of awkward interactions between Google’s paid and organic listings.

Although searchers are growing savvier about which page elements are sponsored and which are not, an Internet user’s primary concern is time: a paid click is the same as a nonpaid click if it leads to the same place.

All of which causes a skeptic to wonder–perhaps, unfairly–whether Sitelinks is a proto-paid-inclusion program for brand keywords (an achievement Yahoo’s soon-to-be-extinct Search Submit Pro could never achieve). Or are Ad Sitelinks a valuable brand-protection tool, and just one more attempt by Google to keep grumpy advertisers satiated?